GILLIS, J. A.*; WITTEN, P. E.; HALL, B. K.; Dalhousie University, Halifax, Nova Scotia, Canada; Zoologisches Institut, University of Hamburg, Germany; Dalhousie University, Halifax, Nova Scotia, Canada: Chondroid bone and secondary cartilage contribute to apical dentary growth in juvenile Atlantic salmon, Salmo salar Linnaeus (1758)

During their spawning migration, anadromous male Atlantic salmon (Salmo salar) experience dramatic cranial alterations, the most remarkable of which is the development of a hook (kype) on the lower jaw. The kype skeleton forms at the distal end of the dentary, and is composed of chondroid bone, a tissue that demonstrates characteristics of both bone in cartilage. In this study, the microstructure and tissue composition of the dentary bone in juvenile Atlantic salmon (parr) was examined using a variety of histological and whole-mount techniques to determine if chondroid bone is present at this life history stage and, if so, to assess its distribution with respect to sex and sexual maturity. While no kype is present in parr, isogenic clusters of chondrocytes of periosteal origin were observed secreting small amounts of pericellular cartilage matrix within the bone matrix at the distal end of the dentary in all parr. These characteristics are highly indicative of secondary chondrogenesis, and suggests that the apical part of the dentary bone in Atlantic salmon does not grow via “pure” intramembranous ossification, but rather via a modified mode of periosteal ossification involving secondary cartilage and chondroid bone. Furthermore, the unusual mode of gender-related dentary growth in adult male salmon (kype formation) could be the continuation of a general mode of salmon apical dentary growth.